In this chapter we shall consider evidence relating to the inheritance of intellectual abilities in man and the lower animals. We shall look first at the meaning of the term "intelligence" and the ways of measuring it; second, at studies purporting to show its inherited components; third, we shall discuss the relation of inherited intelligence to other psychological and physiological variables; and finally, we shall present several theories that have been formulated on the genetic mode of transmission of intelligence and attempt to show their adequacies and inadequacies in the light of existing data.
Although intelligence is a widely used term, it is one that most psychologists find difficult to define satisfactorily. Interest in it can be traced far back in history--like many other problems, at least to the Greeks. Until about the nineteenth century, attention was directed to intelligence largely as a special attribute of man and as a power enabling him to know reality. The primary orientation was metaphysical and epistemological rather than empirical. Thus for Aristotle and later for medieval thinkers, intelligence was a special faculty pertaining to the essence of man as opposed to lower animals and was the means by which he could abstract from sensory data and arrive at concepts. Little attempt was made to look for differences between individuals in this respect, since all possessed it by virtue of being human.
Scientific interest in individual difference in mental ability can be dated from Sir Francis Galton, who later came to be known to some as the "father of mental testing." Galton did not go to any lengths to define intelligence exactly, much less measure it adequately. But he